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Comment by mmbop | Hacker News Reader

Is there a problem of scaling this up to say a 20kg payload?

I’m not an aeronautical engineer, so I guess what I’m asking is if there is some problem scaling up flying machines in an extremely thin atmosphere?

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One issue might be rotor span. Ingenuity has pretty big rotors to counter the thin atmosphere (about 4 feet top-to-tip).

On earth rotor sizes are limited by the speed at the wing-tip. Once you make the rotor too long the tips start approaching supersonic speeds, giving you all kinds of weird mach effects. To make matters worse, the speed of sound is about 30% lower on Mars compared to near earth's surface.

Yea good point. Apparently the blade tip speed on Ingenuity is Mach 0.6-0.7!

Interesting note about this: the speed of sound on Mars is only ~70% of that on Earth, due to less atmospheric density. Might change your Mach numbers!

It's not due to less density, but rather a different gas composition (CO2 vs. N2+O2).

Let's run the numbers!

The speed of sound in an ideal (calorically perfect) gas is given by

  a = sqrt( gamma * R * T )

where gamma is the ratio of specific heats (thermodynamic property of a gas, which may vary with temperature), R is the individual gas constant, and T the temperature of the fluid. All of these are going to be different on Mars versus on Earth:

  Earth:
  R = R_atm = 287 J / (kg * K)
  gamma = 1.44
  T = 293 K (taking room temperature as an average temperature)

  Mars:
  R = R_CO2 = 188 J / (kg * K)
  gamma = 1.37
  T = 210 K (from a quick google, about -60 deg C)

If the Martian and Earth atmospheres were at the same temperature, then the speed of sound on Mars would be 80% that of the speed of sound on Earth. Given the temperature difference, the speeds of sound are

  a_mars = 232 m/s
  a_earth = 347 m/s

So yes, much of the difference is due to the composition: the Martian atmosphere has a higher atomic weight, which leads to a lower individual gas constant, and decreases the speed of sound. However, a substantial amount of the difference is simply due to the different temperatures on the surfaces of the two planets.

I included it

Oh interesting! I can see how that would be a harder problem (although not insurmountable since some planes on earth have supersonic propellers).

Just a quick edit - wow, u didn’t realize the span was already 4ft! Anything much larger could definitely be hard to pack inside a fairing!

What about multiple smaller rotors? Or would that cause weird turbulence effects? Could we use some kind of jet engine?

Here's a paper that describes what the next gen could/should be. The lead author is the head of Mars heli, IIRC.

https://ieeexplore.ieee.org/abstract/document/9843501

In short: 30kg heli, 5kg payloads. Other designs by collaborators are closer to 20kg. It's probably possible to transport a few of these on the existing lander technology, which would be awesome.

The scholar.google.com keywords you want are "Mars Science Helicopter" and a good touchpoint author is T. Tzanetos or S. Withrow-Maser

Ames and JPL were still collaborating on this when I worked there.

I don't know the answer to your question, but for context here are the weights of Mars rovers:

Sojourner (1997): 11 kg

Spirit & Opportunity (2004): 185 kg

Curiosity (2011): 899 kg

Perseverance (2020): 1,025 kg

I know these are much larger - I’m just really curious about the dynamics of scaling up rotorcraft & why it is problematic. ie - do rotor physics become impracticality large or fast at some point for materials science, or is it purely a space problem for rocket launches.

Oh for sure, it's a great question.

I have led efforts building Chinook style tandem rotors with 2 sets of blades from a size like the Trex 800, powered by a 2 stroke engine, as well as 40kg max takeoff quad-planes with both electric quadrotor and 2 stroke engine (for forward motion).

But because I was the main lead and pushing the pace so fast, I wished I did it with a more rigorous aero-engineering to it. I started both projects with barely any experience developing aircraft.

Thinking about your question, here are my 2 cents:

The biggest thing I stugged with is how the vibrations and the accompanying harmonics on the sytem as the rotors spin up and down. I could see it on the logs as the rotors spin through certain Hz, there's would spikes in virbational ampiltudes at predictable frequencies. As the blades get bigger the forces (probably) goes up. Sometimes, these frequecies (especially the lower ones) are at the range where its very hard to find the right materials to damp it out of the control and sensing electronics. Ingenuity probably deals with a virbration range that well into the hundreds/thousand of Hz and I do remember that renge is not a difficult range to damp out, vis a vis the low tens of hertz.

Also, the harmonics is related to ground resonance. I had built my tandem with "skids" that are rigidly attached to the rest of the frame. When the system made contact with the ground on just one skid, that one skid becomes a pivot, the vibration has no where to go and I witnessed first hand, first time, what ground resonance can do to mechanical systems. I can never forget seeing M4 through to M8 hex holt beads being sheared clean off after the resonance event. Only later did I find out that in full scale systems, they have dampeners between the main body and the skids of the aircraft. See https://m.youtube.com/watch?v=IIC-oBzLYhQ ;

Staying in flight is not as hard. But getting the ssytem to land and spin down proerly was big pain without understanding ground resonance and its effect on mechanical design. When I saw the little puny legs of Ingenuity, this experience of mine came into mind and I was glad they had legs like to damp out vibrations as it came down to land.

Then there is the relation between the mechanical vibration regimes of the system, the polilng rates of the foundational flight sensors and the freqency of the main flight stability and movement control loop itself.

With bigger systems, the cables (for signal and power) could run longer too (becoming long long antennas), which means you can run into problems with noise of various origins. If I'd do it again, something like CAN bus would probably be something I look at seriously. Bigger systems also draws more power, and that can have an impact on how much management is needed for noise. Bigger power draw usually also means heavy power store & delivery system, which affects CG management, when then means you can't move things around to management noise. At some point, I felt like I was doing dancing a multi-factorial show.

I wished I could be clearer. Perhaps someone more qualified can chime in.

These must be the masses of the Mars rovers, the weights would be measured in newtons (or pounds in the USA) and would differ between mars and earth.

In a thin atmosphere, lifting a heavier payload needs bigger rotors or increased RPM, which increases power demands and structural stress. The challenge is to keep the vehicle light enough to fly while also making it sturdy enough to carry the payload and survive environments.

Why can’t “more rotors” be another solution?

For sure, that’s another way to decompose “bigger rotors”. It would probably be appropriate to dive into a conversation about Ingenuity’s design goals, requirements, and the trades performed to end up with what they got.

This is what I'm thinking too. A number of posts above talked about physical limits based on speed of sound and rotor length. Cool, so add two rotors, or four.

Ok - this makes sense to me. Also taken into account the context that anything going to space needs to be light at this point in time. Hopefully we don’t have that restriction forever :)

It scales up to 20kg, yes.

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